As cameras and camera sensors become increasingly ubiquitous, there are increasing demands for low-power and high-resolution image sensors. For example, such devices are extremely desirable for hand-held or wearable gadgets, and might be mandatory in swallowable medical devices due to power and heat dissipation constraints. Over the past several decades, remarkable progress in image sensor power reduction and resolution improvement has been achieved by exploiting novel circuit techniques and utilizing increasingly advanced fabrication technologies. Nevertheless, the image capture and processing flow has largely remained the same. The image information is first captured by sensor pixels in an analog format. Then, each pixel output is converted into digital data by analog to digital converters (ADCs). Thereafter, the digital data is compressed, processed or transmitted. The ADC operation is relatively power hungry in the image capturing process. As such, as the number of ADC operations has increased dramatically with the relentless improvement of image resolution, further reducing the power consumption of high-resolution image sensors has become increasingly challenging.
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